1. Field of the Invention
This invention relates to gas generant or propellant compositions, generally in pellet, tablet or wafer form, which are burned at relatively low temperatures to provide nitrogen rich gas to inflate automobile air bag restraint systems. More particularly, this invention relates to improved gas generant compositions including a fuel for producing the nitrogen rich gas, especially non-azides, and a novel oxidizer therefor comprising an inorganic compound having a poly(nitrito) transition metal complex anion.
Though the gas generant or propellant compositions of this invention are especially designed and suited for creating nitrogen-containing gas for inflating passive restraint vehicle crash bags, they would function equally well in other less severe inflation applications, such as aircraft slides and inflatable boats; and more generally, would find utility for any use where a low temperature, non-toxic gas is needed, such as for a variety of pressurization and purging applications, as in fuel and oxidizer tanks in rocket motors; for various portable and military equipment and operations where a storable source of gas is needed.
2. Description of the Prior Art
Automobile air bag systems have been developed to protect the occupant of a vehicle, in the event of a collision, by rapidly inflating a cushion or bag between the vehicle occupant and the interior of the vehicle. The inflated air bag absorbs the occupant's energy to provide a gradual, controlled ride down, and provides a cushion to distribute body loads and keep the occupant from impacting the hard surfaces of the vehicle interior.
The use of protective gas-inflated bags to cushion vehicle occupants in crash situations is now widely known and well documented. In early systems of this type, a quantity of compressed, stored gas was employed to inflate a crash bag which, when inflated, was positioned between the occupant and the windshield, steering wheel and dashboard of the vehicle. The compressed gas was released by the action of actuators or sensors which sensed a rapid change in velocity of the vehicle during a rapid impact, as would normally occur during an accident. Because of the bulk and weight of such stored, compressed gas systems, their generally slow reaction time and attendant maintenance difficulties, these type systems are now largely obsolete, having been superseded by air bag systems utilizing a gas generated by chemical gas-generating compositions. These advanced systems involve the use of an ignitable propellant composition for inflating the air cushion, wherein the inflating gas is generated by the exothermic reaction of the reactants which form the propellant.
The most common air bag systems presently in use include an on-board collision sensor, an inflator, and a collapsed, inflatable bag connected to the gas outlet of the inflator. The inflator typically has a metal housing which contains an electrically initiated igniter, a gas generant composition, for example, in pellet or tablet form, and a gas filtering system. Before it is deployed, the collapsed bag is stored behind a protective cover in the steering wheel (for a driver protection system) or in the instrument panel (for a passenger system) of the vehicle. When the sensor determines that the vehicle is involved in a collision, it sends an electrical signal to the igniter, which ignites the gas generant composition. The gas generant composition burns, generating a large volume of relatively cool gaseous combustion products in a very short time. The combustion products are contained and directed through the filtering system and into the bag by the inflator housing. The filtering system retains all solid and liquid combustion products within the inflator and cools the generated gas to a temperature tolerable to the vehicle passenger. The bag breaks out of its protective cover and inflates when filled with the filtered combustion products emerging from the gas outlet of the inflator. See, for example, U.S. Pat. No. 4,296,084.
The requirements of a gas generant suitable for use in an automobile air bag are very demanding. The gas generant must burn very fast to inflate the air bag, for example, in about 30 milliseconds or less, but the burn rate must be stable, controllable and reproducible to ensure bag deployment and inflation in a manner which does not cause injury to the vehicle occupants or damage to the bag.
The gas generant must be extremely reliable during the life of the vehicle (ten years or more). Ignition must be certain, and the burn rate of the gas generant composition must remain constant despite extensive exposure of the composition to vibration and a wide range of temperatures. The gas generant is protected from moisture when sealed in the inflator, but should still be relatively insensitive to moisture to minimize problems during manufacture and storage of the gas generant and assembly of the inflator, and to ensure reliability during the life of the air bag system.
The gas generant must efficiently produce cool, non-toxic, non-corrosive gas which is easily filtered to remove solid or liquid particles, and thus to preclude injury to the vehicle occupants and damage to the bag.
It follows then that the most desirable atmosphere inside an inflated crash bag would correspond in composition to the air outside it. This has thus far proven impractical to attain. The next best solution is inflation with a physiologically inert or at least innocuous gas. The one gas which possesses the required characteristics and which has proven to be the most practical is nitrogen.
The most successful to date of the prior art solid gas generants which produce nitrogen that are capable of sustained combustion have been based upon the decomposition of compounds of alkali metal, alkaline earth metal and aluminum derivatives of hydrazoic acid, especially sodium azide. Such azide-containing gas generants are disclosed in, for example, U.S. Pat. Nos. 2,981,616; 3,741,585; 4,062,708; 4,203,787; 4,243,443 and 4,547,235.
There are some disadvantages, however, to the use of azides in gas generant compositions used for inflating air bag systems. For instance, sodium azide is a Class B poison and is a highly toxic material. It is easily hydrolyzed, particularly during typical wet slurrying preparation, forming hydrazoic acid which acid is not only a highly toxic and explosive gas, but also readily reacts with such metal ions as Mg, Ca, Mn, Fe, Cu and Pb to form extremely sensitive solids that are subject to unexpected ignition or detonation. See, for example U.S. Pat. No. 5,019,220. Especially careful handling in the manufacture, storage and eventual disposal of such materials is required to safely handle them and the azide-containing gas generants prepared from them.
A number of approaches to a non-azide nitrogen gas generant have been investigated in the prior art as disclosed, for example, in U.S. Pat. Nos. 3,055,911; 3,348,985; 3,739,574; 3,912,561; 4,369,079 and 4,370,181. Many of the prior art nitrogen-containing gas generants that have been reported are based upon nitrogen-containing organic compounds, such as those derived from the various hydroxylamine acid and hydroxylamine derivatives, while others consist of various polymeric binders, hydrocarbons and carbohydrates which are oxidized to produce non-corrosive and, often termed, "non-toxic" gases. However, there some disadvantages to the use of such non-azide fuel materials. For example, the gas products produced from burning these type fuels sometimes produce unacceptably high levels of carbon monoxide, carbon dioxide and water for their use in automobile air bag applications.
Typical oxidizers conventionally used in prior art gas generant compositions are: (1) ammonium nitrate, and alkali and alkaline earth metal nitrite and nitrates, such as KNO.sub.3 and Sr(NO.sub.3).sub.2, (2) ammonium oxalate and various metallic oxides, mixed oxides and bi-metallic complex oxides, based on such cations as Al, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Pd, Sn, Ce, Ta, W and Pb, (3) various metallic halides, including fluorides, chlorides and iodides, such as FeF.sub.3 and CrCl.sub.3, as well as various organic chlorides and iodides, (4) ammonium perchlorate, and alkali metal chlorates and perchlorates, such as KClO.sub.4, (5) various inorganic sulfides, such as MoS2, and sulfur and (6) various other inorganic peroxides, permanganates, chromates and dichromates; as exemplified in U.S. Pat. Nos. 2,981,616; 3,468,730; 3,741,585; 3,719,604; 3,734,789; 3,814,694; 3,898,112; 3,904,221; 3,909,322; 3,931,040; 3,947,300; 3,996,079; 4,062,708; 4,203,787; 4,243,443; 4,370,181; 4,376,002 and 4,734,141.
In contrast to the above discussed prior art, it has now been discovered that gas generant compositions can be improved by using inorganic compounds containing poly(nitrito) transition metal complex anions as oxidants for the gas producing fuel material, whether azides or non-azides, especially the latter.